How To Redecorate Bare Bones Place Without Upsetting Roommate?

Q: I've recently moved into an apartment with a roommate (after having lived alone for a little while). She has been living there for 2 years and needed someone to join the lease. The apartment is clean and has a lot of great potential and my room is cute and private. The common areas, though, are severely lacking.

There is nothing hung on any wall; not one curtain in a window, not one picture on a mantle. The living room has a couch and a table and nothing more. The bathroom has mismatched window curtains and mats and shower curtains. There is no decoration whatsoever, it's all just "functional." We're splitting the rent 50/50 and I'd really like to spruce the place up a bit and add some life into it, but I'm not sure how to broach the subject - she seems perfectly content in with it all bare and mismatched. I'm chomping at the bit to hit up West Elm and Container Store and give this place a little TLC. What's the best way to ask if I can decorate without offending her non-style and causing a rift?